sega/powervr2.cpp: move renderer to threaded code

[angelosa]

• apply portions of PR completely unsafe Naomi / PVR threading hack #7746 to newest code [David Haywood]

[angelosa]

Opening for easier tracking.

List of things denoted here:

• ssr throws garbage GFXs if -nothrottle is used.
• onnaaska draws corrupted frames for main player sprite, needs a sane way to expose the ISP render cycles my first guess.
• mslug6/sfz3ugd/doa2m throws a flickerfest

[ghost]

-nothrottle will break things if the code isn't threadsafe (which the code in my PR was not) as you don't know exactly what set of data the 2nd core is working with. It will differ on other users's machines depending on performance and how busy other cores are too.

[angelosa]

• mslug6/sfz3ugd throws a flickerfest

They separate multiple start renders in small chunks for every frame lifecycle. That will cause a mess because by the time core start to render it's already picking up a different list.